We propose a multi-mode bar consisting of mass elements of decreasing size for the implementation of a gravitational version of the photo-electric effect through the stimulated absorption of up to kHz gravitons from a binary neutron star merger and post-merger. We find that the multi-mode detector has normal modes that retain the coupling strength to the gravitational wave of the largest mass-element, while only having an effective mass comparable to the mass of the smallest element. This allows the normal modes to have graviton absorption rates due to the tonne-scale largest mass, while the single graviton absorption process in the normal mode could be resolved through energy measurements of a mass-element in-principle smaller than pico-gram scale. We argue the feasibility of directly counting gravito-phonons in the bar through energy measurements of the end mass. This improves the transduction of the single-graviton signal, enhancing the feasibility of detecting single gravitons.

The photo-electric effect was a historic milestone in the development of quantum theory, revealing the first evidence of discrete energy of the electromagnetic field through hallmark signatures such as the threshold frequency, intensity-independent energy transfer, and the near instantaneous ejection of photo-electrons. Here, we discuss the photo-electric effect through the lens of semi-classical, quantum, and neo-classical models. We provide a pedagogical outline for how two coupled harmonic oscillators, under a beam-splitter interaction, can exhibit hallmark signatures analogous to the photo-electric effect, including resonance conditions and quantised energy absorption. We discuss the implications of this model for recently proposed graviton detection protocols. This further clarifies that a gravitational version of the photo-electric effect, modelled as discrete energy transfer between harmonic oscillators can provide the first evidence of the graviton.

In a consistent description of the quantum measurement process, whether the wave or particlelike aspect of a system is revealed depends on the details of the measurement chain, and cannot be interpreted as an objective fact about the system independent of the measurement. We show precisely how this comes to be in the measurement of gravitational radiation. Whether a wave or particlelike aspect is revealed is a property of the detector employed at the end of the quantum measurement chain, rather than of the meter, such as a gravitational-wave (GW) antenna or resonant bar, used to couple the radiation to the detector. A linear detector yields no signal for radiation in a Fock state and a signal proportional to the amplitude in a coherent state—supporting a wavelike interpretation. In contrast, the signal from a detector coupled to the meter's energy is nonzero only when the incident radiation contains at least a single graviton. Thus, conceptually simple modifications of contemporary GW antennae can reveal wave-particle duality in the measurement of gravitational radiation.

The quantization of gravity is widely believed to result in gravitons – particles of discrete energy that form gravitational waves. But their detection has so far been considered impossible. Here we show that signatures of single graviton exchange can be observed in laboratory experiments. We show that stimulated and spontaneous single-graviton processes can become relevant for massive quantum acoustic resonators and that stimulated absorption can be resolved through continuous sensing of quantum jumps. We analyze the feasibility of observing the exchange of single energy quanta between matter and gravitational waves. Our results show that single graviton signatures are within reach of experiments. In analogy to the discovery of the photo-electric effect for photons, such signatures can provide the first experimental clue of the quantization of gravity.